nitrogen6x: config: add CONFIG_CMD_MEMTEST

Enable the 'mtest' command on Nitrogen6x and SABRE Lite boards.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
diff --git a/include/configs/nitrogen6x.h b/include/configs/nitrogen6x.h
index d505191..82dc0fc 100644
--- a/include/configs/nitrogen6x.h
+++ b/include/configs/nitrogen6x.h
@@ -382,6 +382,7 @@
 #define CONFIG_CMD_BMP
 
 #define CONFIG_CMD_TIME
+#define CONFIG_CMD_MEMTEST
 #define CONFIG_SYS_ALT_MEMTEST
 
 #define CONFIG_CMD_BOOTZ